The ecological environment of oasis cities in arid regions is extremely fragile. Accurately understanding the spatiotemporal changes in the ecological environment of such cities due to urbanization is of great significance. Karamay, Xinjiang, China, which is an oasis city in an arid region, was selected as the research object in this study. It is located in the most vast and flat gobi desert; this area has become a gathering point of the petroleum and petrochemical industry, which has greatly stressed the fragile ecological environment. LandsatTM/ETM+/OLI remote sensing images from four time periods (2001, 2006, 2011, and 2016) were collected, and a remote sensing ecological index was used to reflect the ecological environment. The variation degree, ecological niche, and gravity center methods were adopted to model the variation, spatial distribution, classification, variation characteristics, and gravity center transfer of the ecological environment. The following conclusions were obtained. (1) The remote sensing ecological index was not high in the study area and fluctuated approximately 0.35. The ecological index values were generally low. (2) Changes in the ecological index grade mainly manifested as rises and falls with few sudden increases. The areas of different types of remote sensing ecological index domains varied significantly among different time periods. (3) The changes in the ecological niche from low to high showed an obvious upward trend. This indicates that a higher ecological index level represents a higher ecological niche; in other words, better ecological environment quality improves the ability to obtain resources. However, the niche values of different types of remote sensing ecological indices were all small with a maximum value of 0.3677. Thus, there remains much space for improving the niche of different types of ecological indices. (4) The center of gravity of different types of remote sensing ecological indices shifted to different degrees. The regions with a high ecological index had strong spatial mobility with an average distance of 42.60 km, whereas regions with a low ecological index were relatively stable spatially. This study explored the temporal and spatial variations in the ecological environment due to human activities and found that urban farmland, park vegetation, residential greening, water, and other elements play a significant role in maintaining a good ecological environment. These results demonstrate humans’ ability to protect the environment. However, there was serious ecological deterioration around the G217 National Highway, which reflects the ecological damage caused by human activities. The above conclusions show that the acceleration of urbanization and implementation of environmental protection need not be in conflict. Understanding the relationship between urban development and environmental protection is significant for the sustainable development of oasis cities in arid regions.